Process for making steel wire



United States Patent Olhce 3,458,365 Patented July 29, 1969 int. Cl. C21d 9/64 US. Cl. 148--12.4 11 Claims This invention relates to a process for making steel wires and to a new product produced thereby. More particularly, the invention relates to an improvement in the patenting heat treating of steel wire as well as to a process for making thin gauge steel Wire having superior properties.

Eutectoid or hyper-eutectoid steels (the latter having 0.75% carbon or higher) have long been used as drawn wire to make high-strength springs and wire ropes. An increasing demand for steel with higher strength resulting from use in new applications has prompted the development of steel wire with still higher strength. Some of these new uses include rocket-case wrapping wires and missileguiding wires.

When steels suitable for use in high-strength wire applications are drawn into thin gauge, scattered brittle spots may be found where structural discontinuities have caused internal damage. One thing responsible for the internal damage is the presence of large pieces of proeutectoid ferrite or carbide. These unwanted structures and the brittleness caused by them in a drawn wire typically result from the conventional patenting heat treatment used.

As is known in the wire industry, patentin heat treating is a thermal treatment applied to rods and wire having a carbon content (in weight percent) of 0.4% and higher. The object of patenting is to obtain a structure which combines high tensile strength with high ductility so that the wire is able to withstand heavy drafting (i.e. reduction in diameter by drawing) to produce the desired finished sizes with a combination of high tensile strength and good toughness.

There are several methods of patenting, all of which are conducted as a continuous process. Typically, the work is heated to a temperature above the upper critical temperature and then cooled through the critical temperature to a temperature level at which the transformation will yield the desired microstructure and mechanical properties. After heating, the wire or rod is quenched, usually by immersion into a molten metal or salt bath of controlled temperature. During the time the wire travels between the heat treating furnace and the quenching bath, it is air cooled. This precooling in air typically results in a rejection of the pro-eutectoid structures and network ferrite forms in surface layers which are always lightly decarburized. In hyper-eutectoid steels, network carbide will form in the material. The heat build-up which occurs at the entry end of the quenching bath also promotes formation of the pro-eutectoid products.

The present invention provides an improvement in the conventional practice as well as a new process for making thin gauge steel wire. The improvement in the conventional practice comprises passing the wire from a heat treating furnace into a vigorously agitated quenchant, without substantial air cooling of the wire, to cool the wire to below the Ar temperature region. The Ar temperature zone is the region at which there is rapid transformation upon cooling in plain carbon steels. This suppresses the formation of pro-eutectoid ferrite on the surface layers of the wire. After quenching as described, the wire is reheated to 750 F. to 1050 F. to obtain isothermal transformation to at least one of upper bainite and fine perlite while avoiding the formation of pro-eutectoid reaction products. Following reheating, the wire may b passed through a holding furnace in which the temperature attained during reheating is maintained for sufiicient length of time to affect complete transformation. In the preferred embodiment, quenching as described above is performed by passing the heated wire through cascading quenchant. The reheating step is also preferably performed in a cascade quenchant. In the first cascading quenching chamber, the quenchant is maintained at a temperature sufficiently below the Ar temperature zone to affect rapid cooling of the work. The quenchant in the second cascading chamber actually reheats the work and maintains it at the higher temperature level. When the size of work and processing rate require, a holding furnace may also be used. Desirably, the temperature achieved in the second quenching chamber is that at which the desired mechanical properties will be achieved, and when necessary, the holding may be employed to maintain the latter temperature until complete transformation is obtained.

Although generally all conventional steel wire may be treated in accordance with the invention to improve properties and minimize embrittlement due to structural deficiencies, it has been found to be particularly well suited to hyper-eutectoid steels and to steels having a controlled low maximum nitrogen, phosphorus, sulfur and silicon contents.

Various tactical missiles under development are guided to their targets by filamentary wires. These wires must have strengths in the 550 to 600K s.i. range. The cold work needed to impart such strength to the wire generally results in embrittlement which cannot be tolerated in this application. Carbon steels well above the eutectoid and usually from about 0.9 to 1% carbon are used to reduce the amount of cold work needed to reach the desired strength. It is also desirable to reduce impurity contents (nitrogen, phosphorus, sulfur and silicon) to improve the over-all ductility. Both of these refinements combine to render the steel difiicult or impossible to patent heat treat acceptably by conventional patenting practice. This is because a very high carbon content results in a network of pro-eutectoid carbides. The massive carbides make drawing impossible, or if the steel can be drawn, the carbides result in structure damage which may not be rectified in subsequent heat treatment. The extent to which the proeutectoid products develop is evident from the microstructure. The product of conventional practice possesses pro-eutectoid carbides that cause brittleness when the particles are large. A characteristic of products which may be produced in accordance with the invention is ductile mode fracture substantially throughout the length of the work, indicating the absence of pro-eutectoid reaction products and the avoidance, during processing, of dynamic strain aging which would, if it occurred, cause fissures and ruptures around hard particles such as inclusions in the work. This latter is avoided in a preferred embodiment by avoiding heat build-up during drawing which might result in dynamic strain aging, for example, by limiting drafts to less than about 22% reduction. The eXpresion dynamic strain aging as it is used refers to the embrittlement occurring during drawing caused by heat build-up. The term ductile mode fracture as used herein refers to the type of fracture typical of ductile steel wherein the fracture area is characterized by a necked-down cup and cone region. Thus, a product may be produced by practicing the invention which has a tensile strength of at least 550K s.i. and a ductile mode fracture substantially throughout the length of the work. Moreover, these properties can be obtained in a wire of a diameter as thin as 0.010-inch.

The rapid reactions in the Ar' transformation during conventional heat treatment make it impossible to suppress pro-eutectoid ferrite formation in the surface layers in the wire during the last patenting process of conventional practices prior to final drawing. These layers are made slightly hypo-eutectoid by carbon loss at the various stages in the processing. The ferrite that forms therein also contributes to brittleness in the wire. These results are in startling contrast to that obtained by practicing the improved method according to the invention.

The following examples will serve to illustrate the practice of the invention. In the following example, a high carbon steel rod or wire having a composition within the following range C 0.90/l.00 Mn GAO/0.50 P 0.005/max. S 0.005/max. Si 0.10/maX. N 0.002/max.

is heated to about 1600 F. in a heat treating furnace. Any suitable furnace may be used; however, the one preferred contains tubes through which the wire travels in an atmosphere of cracked ammonia having a dew point of minus 55 to 70 F., to prevent decarburization. After heating, the wire is passed into a closely situated cascade quenching chamber in which the temperature of the workpiece is reduced to below the Ar temperature zone. The quenchant used is a eutectic NaNO -NaNO composition; however, other suitable quenchants may be used such as lead-bismuth low-melting alloys or other low-melting salt mixtures. The quenching temperature is desirably maintained within the range of 400 to 700 F., preferably at about 550 F. After traveling through the first cascade quenching chamber, the workpiece passes through a second cascade quench in which the temperature is maintained between about 750 and 1050 F., preferably around 975 F., and the work is reheated to this level. The upper limit is selected to avoid producing coarse perlite. Fine perlite, i.e. perlite produced at temperatures less than about 1050 F., may sometimes be desirable. The same quenchant may be used in both quenches to avoid the problems of mixing or contamination of the quenchants. From the second cascade quenching chamber, the workpiece is passed into a holding furnace in which the temperature is held at the level required to develop the desired properties and for a sufiicient time so that complete transformation occurs. It is convenient and most efficient to have the temperatures adjusted so that the holding fur nace maintains the workpiece at the same temperature as that of the second quench. The wire in the holding furnace is finally collected on a take-up reel.

To produce ultra-highstrength wire (i.e. wire with yield strength greater than 550K s.i.) and with a gauge of 0.004 or 0.005-inch, several repetitive treatments as described above may be employed. The first or rod treatment may not require the holding zone, but the use of this zone may be desirable for subsequent patentings. For wire products with gauge of less than 0.015-inch, the final drawing is desirably performed completely submerged. To produce a wire of this thin gauge without ductile mode fracture substantially throughout the length thereof (i.e. 100 ft. increments) it is necessary to draw the wire under controlled conditions, e.g. speed and draft, so that dynamic strain aging causing fissures and ruptures is avoided.

The use of a vigorously agitated quenchant as in the cascade quenches described above provides a more positive cooling than is possible with the usual splash-type fine wire drawing machines. Because the steel remains generally cooler during drawing, there is less opportunity for ductility loss due to dynamic strain aging. Modern fine-wire machines generally employ splash lubrication, with the water-base lubricant jetted against the wire at each die, in a chamber filled with sprays of the cooling lubricant. In spite of all the precautions taken, some build-up of heat occurs which as just mentioned may C 0.90/ 1.00 Mn DAG/0.50 P 0.005/max.

S 0.005/max. Si 0.10/max.

N 0.002/max.

was patented by the double cascade quench system described above. It was then drawn in 10 dies three times with 20% drafts, each drawing being followed by full cooling to room temperature before the next draft. The wire was patented by the double cascade quench system and drawn five times in the same manner as before. The third patenting treatment was followed by seven drafts and the fourth by nine drafts at which point the wire diameter was about 35 mils. After the final or fifth patenting, a metallic copper coating was applied. Final drawing was performed submerged and was accomplished with small angle dies taking 15 drafts and at a finishing speed of about 500 f.p.m. to prevent heat build-up in the submerged drawing. The wires thus produced had a final gauge of 5 mils and strengths ranging from 570 to 650K s.i. The wire was ductile enough to be woven into screens or fabrics or stranded into fine ropes or strands.

It is apparent from the above that various changes may be made without departing from the invention. It is also apparent that the use of a cascade quenching in accordance with the improved process described permits bringing the quenchant to within several inches rather than several feet of the heating zone. Thus, a relatively uncooled wire enters the vigorously agitated quenching liquid and there is no air-cooling transformation. The provision of this prequench to below the Ar temperature zone ensures an extremely rapid quench through the Ar transformation zone and further reduces the chances of forming pro-eutectoid structures. Cascade quenching as mentioned herein refers to a quenching technique using a reservoir of quenchant and a tray disposed above the reservoir through which the work, e.g. wire, travels in a straight line path. Quenchant is pumped from the reservoir into the tray where it overflows back into the reservoir. Achieving the transformation by reheating, as in the present invention, results substantially in isothermal transformation to upper bainite and/or fine perlite structures while avoiding the formation of pro-eutectoid reaction products as would necessarily result if the work were quenched to the temperature levels necessary to develop mechanical properties.

We claim:

1. In the patenting heat treatment of steel wire or rod wherein the work is continuously heated in a heat treating furnace to above the steels upper critical temperature and cooled through the critical temperature to a predetermined temperature level at which the transformation will yield the desircd microstructure and mechanical properties, the improvement comprising passing said work from said heat treating furnace, without substantial air cooling of said work therebetween, into a closely situated first cascade quenching chamber in which the temperature of the heated work is reduced by a cascading quenchant to below the Ar temperature zone rapidly to suppress the formation of pro-eutectoid carbides and ferrite, and then into a second cascade quenching chamber in which work is reheated to 750 to 1050 F. by a cascading quenchant.

2. An improvement according to claim 1 wherein the temperature of the work is brought to between 400 and 700 F. in the first cascade quenching chamber.

3. An improved method according to claim 1 wherein said work is passed from said second cascade quenching chamber into a holding furnace in which the temperature of the work is held at the temperature attained in the second quenching chamber for a sufiicient length of time for complete transformation.

4. An improvement according to claim 1 wherein said quenchant is a eutectic NaNO -NaNO composition.

5. An improvement according to claim 1 wherein said quenchant is a lead-bismuth alloy.

6. An improvement according to claim 1 wherein said steel is a hyper-eutectoid steel containing up to 0.005% phosphorus, up to 0.005% sulfur, up to 0.10% silicon and up to 0.002% nitrogen.

7. A process for making thin gauge steel wire comprising continuously heating a steel wire or rod workpiece in a heat treating furnace to above the steels upper critical temperature, passing said heated workpiece from said heat treating furnace into a closely situated first cascade quenching chamber in which the temperature of the heated wire is reduced by a cascading quenchant to below the Ar temperature rapidly to suppress the formation of pro-eutectoid carbides and ferrite and then passing the workpiece into a second cascade quenching chamber in which the workpiece is reheated to 750 to 1050 F. by a cascading quenchant, maintaining the temperature of said workpiece within said last mentioned temperature range until substantially complete transformation occurs and drawing said workpiece to reduce the diameter thereof.

8. A process according to claim 7 wherein said drawn work is reheated, quenched and held as described, cooled to about room temperature and then redrawn.

Q A process according to claim 8 wherein said treatment is repeated until the desired gauge is attained.

10. A process according to claim 8 wherein said drawing is performed under controlled conditions so that dynamic strain aging causing fissures and ruptures in the workpiece is avoided.

11. A process according to claim 9 wherein said wire is given a metallic coating and the coated wire is drawn to gauge.

References Cited UNITED STATES PATENTS 2,852,422 9/1958 Hess 148---12.4

L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner US. Cl. X.R. 148143 

1. IN THE PATENTING HEAT TREATMENT OF STEEL WIRE OR ROD WHEREIN THE WORK IS CONTINUOUSLY HEATED IN A HEAT TREATING FURNACE TO ABOVE THE STEEL''S UPPER CRITICAL TEMPERATURE AND COOLED THROUGH THE CRITICAL TEMPERATURE TO A PREDETERMINED TEMPERATURE LEVEL AT WHICH THE TRANSFORMATION WILL YIELD THE DESIRED MICROSTRUCTURE AND MECHANICAL PROPERTIES, THE IMPROVEMENT COMPRISING PASSING SAID WORK FROM SIAD HEAT TREATING FURNACE, WITHOUT SUBSTANTIAL AIR COOLING OF SAID WORK THEREBETWEEN, INTO A CLOSELY SITUATED FIRST CASCADE QUENCHING CHAMBER IN WHICH THE TEMPERATURE OF THE HEATED WORK IS REDUCED BY A CASCADING QUENCHANT TO BELOW THE AR'' TEMPERATURE ZONE RAPIDLY TO SUPPRESS THE FORMATION OF PRO-EUTECTOID CARBIDES AND FERRITE, AND THEN INTO A SECOND CASCADE QUENCHING CHAMBER IN WHICH WORK IS REHEATED TO 750* TO 1050*F. BY A CASCADING QUENCHANT. 